1. Field of the Invention
The invention relates to a light source. More particularly, the invention relates to light sources used in, for example, display systems for projecting images, such as data projectors, and automobile lamps, including headlamps, signal lamps, traffic lamps, etc.
2. Description of the Related Art
Currently, ultra-high pressure mercury lamps having high light collection efficiencies are mainly used in display systems for projecting images, such as data projectors. Recently, however, light sources that do not contain mercury are desired from the viewpoint of environmental protection and other reasons. In particular, light sources are desired that can be used in data projectors (and other systems), and which have a continuous emission spectrum in the visible Light range, a high light collection efficiency, a high lamp efficiency, and a long durability, and which do not contain mercury.
Japanese Patent Laid-Open Publications Nos. Hei 3-152852 and 2000-90880 each disclose a mercury-free metal halide lamp filled with a plurality of different kinds of metal halides and xenon gas. These mercury-free lamps can emit light having overlaid emission peaks corresponding to the different kinds of metal halides. In particular, Japanese Patent Laid-Open Publication No. 2000-90880 describes a mercury-free metal halide lamp using three kinds of halides of sodium, indium, and thallium. In this lamp, the respective amounts of these halides are set so that the absorption spectra of the sodium, indium, and thallium halides occur at 589 nm, 410 nm and 451 nm, and 535 nm, respectively. The mercury-free metal halide lamp exhibiting a continuous spectrum in the visible light range is thus obtained as shown in FIG. 2 in Japanese Patent Laid-Open Publication No. 2000-90880.
The technology disclosed in the specification of Japanese Patent No. 3196649 (corresponding to Japanese Patent Laid-Open Publication No. Hei 9-120800) proposes a mercury-free electrodeless discharge lamp. In this lamp, microwaves generated by a magnetron are guided through a waveguide tube to a rotating discharge bulb, so as to allow a metal halide and a noble gas, both filled in the discharge bulb, to emit light. Japanese Patent No. 3196649 also discloses that use of indium iodide as a metal halide filled in the discharge bulb can result in a continuous spectrum in the visible light range.
The metal halide lamps described in Japanese Patent Laid-Open Publications Nos. Hei 3-152852 and 2000-90880 are designed so as to obtain an emission spectrum by overlaying the emission spectrum peaks of the three kinds of halides. As a result, for example, the lamp disclosed in Japanese Patent Laid-Open Publication No. 2000-90880 provides a continuous spectrum in the visible light range. As is apparent from the spectrum distribution diagram disclosed in FIG. 2 in Japanese Patent Laid-Open Publication No. 2000-90880, however, the metal halide lamps emit light having three large intensity peaks around a blue wavelength of 450 nm, a green wavelength of 540 nm, and a wavelength of 590 nm. The intensities at these peaks are at least two times larger than the ones of other wavelengths. Further, the peak intensities around the green wavelength of 540 nm and the wavelength of 590 nm are at least 1.6 times larger than the peak intensity around the blue wavelength of 450 nm.
The metal halide lamp described in Japanese patent No. 3196649 (corresponding to JP 9-120800 A1) is a discharge lamp of an electrodeless type, in which microwaves generated by an external magnetron are guided through a waveguide tube to a discharge bulb. As described in paragraph No. 0003 of Japanese patent No. 3196649, this metal halide lamp easily couples electromagnetic energy with a halide in comparison to discharge lamps having electrodes (also referred to below as an electrode type), and therefore this type of lamp is easily made mercury-free. Furthermore, since this metal halide lamp has no electrodes, blackening in the discharge space does not occur. However, it is not easy to apply the halide used in the electrodeless type discharge lamp, which is disclosed in this publication, to electrode type discharge lamps. For this purpose, it is desired to solve the problems of coupling electromagnetic energy with halides and blackening in a discharge space.